In 2003, the United States invaded Iraq to topple dictator Saddam Hussein. During the subsequent eight years, the U.S. spent trillions of dollars and lost thousands of American lives fighting an insurgency waged against the new government.

Today, two years after the last American combat forces left Iraq, the country is teetering on the edge of total chaos and threatening to pull the U.S. back into war. The Islamic State (ISIS), with the help of disenfranchised Sunnis, is gaining ground. 

Losing Iraq, a new FRONTLINE documentary airing tonight 10:00 p.m., EST on P.B.S., takes a closer look at how we got to this point and where we're going in the future. 

In this exclusive clip General David Petraeus explains his controversial "Sons of Iraq" initiative, in which he put Sunni insurgents on the American payroll. A critical part of the "Anbar Awakening," the effort was heralded as a turning point in the war against al-Qaeda.